In the manufacture of paper, an aqueous cellulosic suspension containing cellulose fiber and selected mineral pigments is formed into a paper sheet. The cellulosic slurry is generally diluted to a consistency (percent dry weight of solids in the slurry) of less than 1%, and often below 0.5% ahead of the paper machine. Associated with papermaking slurries, called furnishes, is a large variation in the size and shape of the particles present. These particles may range in size from less than one micrometer for many mineral pigments or fillers, up to several millimeters in their largest dimension for fibers. The initial dewatering of a paper furnish typically takes place by the ejection of the cellulosic furnish onto or between filter fabric(s), called the wire. The openings in these wires are typically on the order of 200 mesh, which corresponds to a hole size capable of passing particles with a diameter of 76 micrometers. If no forces of attraction exist between particles, the mineral pigments would very easily pass through the wire and would not be retained in the sheet, compromising the benefits for which the mineral pigments were added. Thus, under normal papermaking circumstances, many components of the furnish that are small enough to pass through the openings in the wire will require modification if they are to remain in the sheet.
As the fibers form a mat on the wire, they generate their own filter medium and many of the smaller particles in the furnish may be trapped by simple filtration in the fiber mat, particularly if the sheet is thick, i.e. high basis weight. However, even if the basis weight is high, a significant fraction of the small particulate material may not be adequately retained. When basis weights are low or machine turbulence prevents mat formation, the filtration mechanism of small particle retention is severely inadequate. Under papermaking circumstances when the filtration mechanism is inadequate, chemical treatments generally called retention aids are required to modify the interparticle interactions thereby resulting in coagulation and/or flocculation of the particles.
Retention of small particulate components leads to numerous benefits for the papermaker. Mineral fillers like clay and calcium carbonate are often less expensive than fibers, and substitution of such fillers for fiber provides a way for the papermaker to reduce the raw material costs. Retention of fillers and fiber fines is also necessary to achieve the sheet properties needed for a given end use. Such properties might include sheet opacity, brightness, and appropriate ink interactions. Because the small particles have large surface areas for a given mass, significant amounts of additives such as dyes or sizing agents can be attached to them making retention of the fines necessary for effective utilization of such additives.
Filler particles and fiber fines which are not retained initially, or in the so called first pass, are to a large extent recycled via the white water system back into the furnish, increasing the fraction of small particles present in the furnish over time. This result is often unsatisfactory for several reasons. Some important and expensive materials lose their effectiveness upon recycling in the white water system, and their retention in the first pass is needed for performance or sheet properties. Examples of such materials are titanium dioxide and alkaline sizing agents. Although the total amount of fines in the sheet may be increased in this way, their distribution in the sheet will tend to be very uneven frequently resulting in two-sided phenomena of the paper. In addition, the concentration of unretained materials in a papermachine's white water system can contribute to deposit problems and related runnability problems which result in lost or slowed production and poor product quality. These problems are remedied by using effective retention aids, resulting in a cleaner machine with improved runnability, more efficient use of fiber and filler raw materials, and less waste to the mill's waste treatment facility.
Typical retention aids include polymeric coagulants, which are cationic solution polymers of low to medium molecular weight (10.sup.3 -10.sup.6 g/mole). Because these are polymers with high cationic charge densities, their activity in attention applications is believed to derive from their interactions with negatively charged papermaking components. Because of the shear sensitivity and relatively small floc sizes often formed with polymeric coagulants, they are seldom used alone as retention aids, but are used in conjunction with a flocculant as a dual polymer program. In this way, the coagulant is thought to provide an initial agglomeration of particles which can be more effectively flocculated.
Similarly, hydrolyzable aluminum salts are used extensively as coagulants in papermaking. Because of the acid generated by the aluminum hydrolysis, the pH of machines using alum is depressed, and the process is referred to as "acid papermaking". The aluminum species possessing the greatest coagulating ability are formed in the pH range of 4 to 6. Polyaluminum chlorides are also effective coagulants. Being partially neutralized, they do not depress the pH to the extent that alum does and are generally more applicable over a wider pH range.
Flocculation describes a number of possible strategies which result in agglomeration of small particles. Different degrees of flocculation are required at each stage of operation in pulp and paper mills. At the forming wire on the paper machine, paper is formed by the rapid dewatering of the furnish. Retention aids operate by flocculating of the components of the slurry before the slurry is consolidated as the sheet in the consecutive dewatering stages. A proper level of flocculation is necessary to provide the required retention and drainage rate.
In a single polymer program, a flocculant, typically a cationic polymer, is the only material added. The flocculent is added to the thin stock after the cellulosic and filling streams have been mixed. Another method of improving the flocculation of cellulosic fines, mineral fillers and other furnish components on the fiber mat is the dual polymer program, also referred to as a coagulant/flocculant system, added ahead of the paper machine. In such a system there is first added a coagulant, for instance a low molecular weight synthetic cationic polymer or cationic starch, to the furnish typically after mixing of the cellulosic and filling streams, for initial agglomeration of such particles, followed by the addition of a flocculent. The flocculant generally is a high molecular weight synthetic polymer. The presence of these large agglomerates in the furnish as the fiber mat of the paper sheet is being formed increases retention. The agglomerates are filtered out of the water onto the fiber mat, whereas unagglomerated particles would to a great extent pass through.
In systems containing high concentrations of anionic polymeric/oligomeric substances, the performance of cationic polymers is often detrimentally affected. These anionic substances may be of inorganic or organic origin. Silicates used as hydrogen peroxide stabilizers in pulping, bleaching, and de-inking processes and species extracted from the wood like polygalacturonic acids and lignin derivatives are the most typical examples of components of anionic detrimental substances, also called "anionic trash". Nonionic polymers are affected by these substances to a much lower degree than cationic polymers.
An example of a nonionic polymer system is the polyethylene oxide (PEO) and cofactor program. This system is often an effective retention aid for newsprint and other mechanical pulp furnishes. Known cofactors include kraft lignin, sulfonated kraft lignin, naphthalene sulfonate, tannin extract, and phenol-formaldehyde resins. A recent EPO patent application (Echt, EPO Application No. 621 369 A1, 1995), discloses using poly(p-vinyl phenol) as a cofactor. Moreover, phenol sulphone formaldehyde resins are described in WO 95/21295 to improve retention. However, these resins are added after a cationic polymer is incorporated into the cellulosic suspension.
Another approach using nonionic polymers in tandem with a cofactor include that of Huinig Xiao and R. Pelton. Xiao reported synthesis of a copolymer of acrylamide and poly(ethylene-glycol) methacrylate. This copolymer contains pendant PEG chains which, as claimed by Xiao and Pelton, are able to interact with resole-type phenolic resin to form the three dimensional structures responsible for its good performance as a retention polymer. However, Xiao and Pelton did not report any beneficial effect from the use of phenolic resin on flocculation performance of polyacrylamide homopolymers. This information is summarized in WO 94/17243 application. Unexpectedly, a synergism was found between homopolymer acrylamide and certain cofactors such as a phenol formaldehyde resin under the appropriate conditions. The strategy is discussed in EPO Publication No. EP 0 773 319 A1.
There is growing interest in increasing the filler content of paper, due to both raw material costs and energy savings, as filled papers require less refining and are easier to dewater than unfilled papers. As used herein, the term fillers includes calcium carbonates, clay in various forms, talc, titanium dioxide, gypsum, hydrated aluminum oxide, silicas, and plastic pigments among others. In addition, final sheet properties can be enhanced by increased filler loading, such as opacity, brightness, and printability.
Attempts to achieve improved filler retention have focused on modification of the filler surface by chemical pretreatment. Commercial filler material suppliers have chemically modified their filler particles by attaching/associating cationic polymers to the surface to be sold as specialty-grade fillers. In a similar way, chemical vendors have added cationic polymers to the filler slurries on-site at a paper mill to induce filler preagglomeration prior to the filler slurry being mixed with the fiber slurry. Such filler pre-agglomeration has been disclosed in PCT Application No. WO 86/04370; U.S. Pat. Nos. 4,295,933 and 4,272,297 and in British Patent No. 2,001,088.
Modified filler formation by treating a filler first with an aqueous colloidal dispersion of cationic melamine-formaldehyde resin followed by treatment with aqueous vinyl alcohol polymer solution is described in U.S. Pat. No. 4,495,245. Surface treatment of cationic fillers with a dispersing agent which is a cationic polyelectrolyte to render the particles cationic is described in U.S. Pat. No. 5,244,542. Enhanced retention is obtained by flocculating beforehand a mineral filler and a binder (such as a starch or synthetic polymer) prior to incorporation into a fiber suspension in U.S. Pat. No. 4,943,349. In these ways, increases in filler retention in the final product have been observed.
U.S. Pat. No. 4,913,775 provides a general overview of modes of addition of treatment agents for the production of paper and paper board. FI 67735 describes a process in which retention is improved by the addition of a cationic polymer and an anionic component which may be pre-mixed; however, that reference states that such an addition procedure does not yield optimum results. U.S. Pat. No. 4,388,150 describes starch and colloidal silicic acid which may be pre-mixed and then added to stock. This reference also states that such a procedure does not provide maximized results.
U.S. Pat. No. 5,670,021 describes a process for papermaking wherein a mixture of an alkali silicate and a phenolformaldehyde resin optionally are added to the filler before addition to the cellulosic slurry. The system is then treated with PEO as the flocculant, to improve retention, drainage and formation in the papermaking process.
The use of nonionic programs involving PEO have several quite significant liabilities. The high molecular weight PEO used for wet end applications is typically a dry polymer which requires an extremely costly and tedious makedown system. If the PEO is not properly prepared the performance of the polymer dramatically suffers not to mention the potential for detrimentally impacting runnability due to particulate residuals in the polymer feed. PEO is known to be chemically sensitive to residual oxidizers and some metal ions which are commonly present on the wet end of papermachines. This molecular weight degradation can result in sporadic, unstable, performance. The performance of the PEO containing programs are known to be very shear sensitive. This sensitivity potentially hinders performance on shear intensive large modern papermachines. Finally, because of the raw materials and processing involved, PEO is quite expensive relative to many other water-soluble polymers.
The present invention departs dramatically from any previous disclosures of improved filler retention based on the chemical pretreatment of filler particles. As defined herein the term filler components includes, but is not limited to conventional fillers as described above, and is meant to encompass small filling and property-modifying solids. The present invention describes the novel application of a dual polymer program for the process of papermaking wherein the flocculant is composed of monomers from a select group. Furthermore, this novel application allows the papermaker selective retention of filling materials relative to other components present in the papermaking slurry. This application strategy does not necessarily need to yield an improvement in overall retention to offer the benefits of improved runnability and sheet properties. This application is designed to preferentially retain certain characteristic fractions of the furnish, in this case fillers. These fillers can be retained preferentially over cellulosic materials or in some cases preferentially with respect to brightness degrading ink particles.
The present invention departs dramatically from previous disclosures of improved filler retention based on the chemical pretreatment of filler particles. This disclosure teaches an improvement to papermaking by pretreating filling materials as opposed to the conventional method of preflocculation of filling materials. Specifically, this invention teaches the use of phenolic additives (enhancers) to pretreat fillers prior to introduction of the filler stream into the cellulosic papermaking slurry for improved retention of filling components and runnability. This addition can occur with or without a pH adjustment using common hydrogen atom donors such as dilute mineral acids although the resulting performance is often significantly increased for the flocculants disclosed in this invention if the pH of the filling slurry is adjusted to about 5. In addition, this invention teaches the ability to maintain the observed benefits even in the 5 presence of high levels of anionic detrimental substances.
In the first step of this invention, a phenolic enhancer material is added to the filling components of choice before the filler slurry is mixed with the fiber slurry, with the phenolic enhancer material either being mixed with the filler slurry, stored for a given amount of time, and fed separately as a product, or with the phenolic enhancer material being mixed with the filler slurry on-site at a paper mill just prior to adding the filler slurry to the fiber slurry. This addition can occur with or without a pH adjustment using common hydrogen atom donors such as dilute mineral acids. In a subsequent step in this invention, a flocculant is added to the furnish that contains the pretreated filler stream resulting in a significantly selective increase in retention of filler relative to other low brightness particles in the final sheet of paper produced. This interaction also leads to improvement in retention, drainage, formation and in general runnability of the papermaking process. Thus, this invention teaches a new application that offers improved levels of retention, formation, uniform porosity, and overall dewatering as well as selective retention of filling solids relative to other particulate materials in the system.